1. Field of the Invention
The present invention relates to an automatic time constant adjustment circuit that finds application in circuits having a time constant, such as filter circuits and delay circuits. More particularly, the present invention relates to an automatic time constant adjustment circuit that automatically adjusts an error in the time constant produced by a resistor-capacitor serial circuit formed within an integrated circuit (hereinafter “IC”).
2. Description of Related Art
In an IC, a resistor is formed typically by diffusing an impurity, and a capacitor is formed typically by forming a thin oxide film on a semiconductor substrate and then fitting a metal electrode thereon (such techniques by which resistors and capacitors are formed within an IC will hereinafter be referred to as IC processes). In these processes, a variation in the amount of diffused impurity, a variation in the thickness of the oxide film, and other factors produce large variations in the resistance of the resistor and in the capacitance of the capacitor.
Specifically, when a resistor and a capacitor are formed within an IC, the “product of the resistance of the actually formed resistor and the capacitance of the actually formed capacitor” typically differs by a maximum of about ±20% from the “product of the design value of the resistance and the design value of the capacitance.” That is, a fabrication error of about ±20% arises in the product of the resistance and the capacitance. Here, the “design value of the resistance” denotes the ideal resistance of the resistor formed within the IC, in other words, the resistance that the resistor will have if the fabrication error is 0%. Likewise, the “design value of the capacitance” denotes the ideal capacitance of the capacitor formed within the IC, in other words, the capacitance that the capacitor will have if the fabrication error is 0%.
Consider a case where a first-order low-pass filter is built with a serial circuit composed of a resistor (of which the actual resistance is Ra) and a capacitor (of which the actual resistance is Ca), both formed within an IC. The time constant of the low-pass filter is given by Ra.Ca. Here, an error of about ±20% in the time constant from the design value thereof results in as large an error (i.e., of about ±20%) in the cut-off frequency 1/(2πCa.Ra) of the low-pass filter from the design value thereof.
According to another conventionally proposed configuration, in a variable-time-constant circuit of which the time constant is controlled according to a setting voltage fed from outside, the error in the time constant resulting from the error in the product of a resistance and a capacitance is automatically adjusted so that the time constant is controlled uniquely according to the setting voltage irrespective of the presence or magnitude of resistance/capacitance errors. Examples of such techniques are disclosed in Japanese Patent Registered No. 2808195 (hereinafter, Patent Publication 1) and Japanese Patent Application Laid-Open No. H7-321602 (hereinafter, Patent Publication 2).
As described above, if a time constant contains an error of about ±20% from the design value thereof, the cut-off frequency of a first-order low-pass filter having such a time constant contains as large an error (i.e., of about ±20%), giving the low-pass filter filtering characteristics greatly different from those desired.
One way to prevent this is to adjust, before factory shipment, a variable resistor or the like provided outside the IC with a view to making the characteristics of the filter closer to those desired (i.e., making the time constant closer to the design value thereof). Disadvantageously, however, such adjustment is not only time- and labor-consuming, but also increases the production cost of the printed circuit board or device that incorporates the filter.
The conventional configurations disclosed in Patent Publications 1 and 2 mentioned above, indeed, permit automatic adjustment of the time constant, but requires, disadvantageously, the supply of a setting voltage from outside. Moreover, the variable-time-constant circuit receives, in the form of an analog voltage, and the error detected in the product between the resistance and the capacitance, and the time constant of the variable-time-constant circuit is adjusted according to this analog voltage. Disadvantageously, this complicates the overall circuit design encompassing the circuit that detects the error and the variable-time-constant circuit, leading to a larger circuit scale and higher power consumption.